When it comes to the local soccer mom and her minivan, she doesn't care whether or not that engine is carbureted or fuel injected. All she cares about is whether or not it'll start and get her to the appropriate location to drop off the kids and have a couple hours of peace and quiet-she'll think about picking them up later-maybe. For years, carburetion was the mainstay of every car's fuel delivery system, and it worked relatively well. That is, until the needle for the float stuck, the carburetor needed a rebuild, or some enterprising hot rodder thought he knew what he was doing, and adjusted it.

It was in the mid-'80s when EFI started to become prevalent on new cars. Sure, there were some car manufacturers that tried a couple of different types of EFI in the '50s and '60s, but it wasn't until the mid-'80s when it really became a mainstay from the manufacturers. Those early EFI setups were new for the times, and more than capable to make any mechanic pull out his hair while troubleshooting an issue. And in the eyes of many hot rodders, some things never change.

Why did the auto manufacturers arbitrarily switch to EFI? Was it a better mousetrap? Was it cheaper? Or did they do it to simply keep the average guy from doing repairs himself, and have to take the car to the dealership for repairs? Whatever you think the reason is, EFI is here to stay, and it's even making the move within the aftermarket for us muscle car guys to adapt to our cars. But what is fuel injection, and how does it work?

What's the Difference?

The primary difference between a carburetor and fuel injection is that fuel injection atomizes the fuel by pushing it (under high pressure), through a nozzle. A carburetor relies on a low pressure signal, or vacuum created by intake air rushing through it to add the fuel to the airstream. Fuel injection generally increases an engine's fuel efficiency by improved cylinder-to-cylinder fuel distribution. In other words, since the fuel is more evenly distributed, less fuel is needed for the same power output. When cylinder-to-cylinder distribution is less than ideal, as with a carburetor, some cylinders usually get an excess of fuel. This is a side effect of making sure that all cylinders receive at least the minimally required amount of fuel. Extreme distribution problems will, if not immediately, eventually begin to reduce efficiency. Overly extreme distribution issues substantially affect power. By optimizing the uniformity of cylinder-to-cylinder fuel-mixture distribution, all of the cylinders reach their maximum power potential, and the engine's overall power output improves.

There are different types of electronic fuel injection systems found in automobiles and other vehicles, and we'll briefly explain them here.

Throttle Body Injection (TBI)

This is possibly the simplest type of fuel injection. A device that resembles a carburetor's "body," is fitted with fuel injection nozzles, and simply replaces the carburetor. For some automakers, TBI injection was the beginning of the more complex multi-point systems that followed. Although the TBI injection system may not be as precise as some of the EFI systems that have followed, TBI meters fuel better than a carburetor, are less expensive, and are easier to service.

Port or Multi-Point Fuel Injection (MPFI)

Multi-point fuel injection means that there is a separate injector nozzle for each of the engine's cylinders. Each injector is located directly outside of an intake port. This is why the system is occasionally referred to as port injection. With the injector spraying the fuel vapor this close to the intake port, it ensures that it will be drawn completely into the engine's cylinder. MPFI has the capability to meter fuel more precisely than TBI design.

Sequential Fuel Injection (SFI)

Sequential fuel injection, also called sequential port fuel injection (SPFI) is a type of multi-port injection. This does not mean that it's the same as MPFI. A basic Multi-Port injection employs multiple injectors, as does Sequential Fuel, but the Multi-Port injectors all spray their fuel at the same time or in groups. As a result, the fuel may pool, or puddle, in an intake port for as long as 150 milliseconds when the engine is idling. Sequential fuel injection triggers each injector nozzle independently. With Sequential injection, the injectors are timed like spark plugs, and they spray the fuel immediately before, or as the intake valve in their respective cylinder opens.

Direct Injection

Direct injection is the most complex and takes the concept of fuel injection about as far as it can go. Direct injection means that the injectors are injecting fuel directly into the combustion chambers, past the valves. While this type of injection was most prevalent in diesel engines for many years, direct injection is starting to make an appearance in gasoline-engine designs. When referring to this type of injection in regards to gasoline power, it's sometimes called DIG for direct injection gasoline. Again, fuel metering is even more precise than in the other injection systems, and Direct Injection gives engineers yet another variable to influence precisely how the combustion occurs in the engine's cylinders.

It's All About the Parts

When it comes to fuel injection, choosing the right engine parts that have to work with the injection system is just as important as it is for choosing them with a carbureted engine. We asked Darren Tedder of Prism Racing in Conyers, Georgia, how important camshaft selection is to an EFI engine. According to him, running a cam with larger than average lift and duration numbers is actually easier to do with EFI than it is a carburetor. With a carburetor, you have to find a balance between wide-open throttle, cruising speed, and idle. Tuning for one aspect is easy, but your setting for one situation can-and usually does-have an adverse affect on the others. For instance, getting an engine that has a camshaft with a large duration number to idle, means raising the idle setting, which in turn opens the venturi butterflies more, which in turn pulls more fuel because of the increased air flow drawn over the idle and transition circuits. Like we said, tuning a carburetor requires concessions with at least one aspect of the fuel curve.

On the other hand, tuning an EFI-equipped engine allows you to infinitely tune the fuel curve. Again, say you have an engine with large camshaft duration and lift, with EFI, you can fine tune the idle mixture, cruise mixture, and wide open throttle mixture simply by telling the injectors to open for a longer or shorter period of time, at any give rpm. Another thing to keep in mind is that EFI is not as affected by a vacuum signal as a carburetor is. When using a carburetor, vacuum from the engine draws fuel into the intake. With EFI, it is squirted under pressure. Since that's the case, unless you're running power brakes, a large amount of vacuum is not required.

When considering an intake manifold for an EFI engine, the same principles apply as if choosing an intake for your carbureted engine. Dual-plane intake manifolds are named for their split plenum opening (where the carburetor mounts). Each side of the opening feeds four cylinders of a V-8 engine. Dual-plane intake manifolds are the most popular for performance street engines, because they generally build power across a wider rpm range, and start making their power at a lower rpm-typically around 1,500. Single plane intake manifolds are aptly named, because they have one large ôhole,ö or opening in the plenum where the carburetor mounts. Fuel from the carburetor/throttle body enters the intake into a single plenum through the single opening with no separation between the right and left side. That single opening feeds all eight cylinders on a V-8. Single plane intakes are typically less restrictive and work best to build power between 3,000 and 8,000 rpm. Because of the higher rpm range capabilities, the single plane intake manifold is best suited for racing applications.

Choices, Choices

OK, enough with the lesson. How does EFI get retrofitted into a muscle car, and which one should you use? The choice comes down to installing what you're comfortable with. If you're comfortable designing and installing a complete Multi-Port system, go for it. There are manufacturers that have intakes and complete kits available to do just that. The tunability and other benefits of such a system are great for the guy that has a firm grasp of tuning EFI. But what if you like the attributes that EFI has to offer, but you're not a guy that has spent a lot of time tuning and installing EFI kits? There are a couple of kits out there that claim they are bolt-on, and we decided to test one and see just how easy it really is.

For our testing, we have heard nothing but good things about the FAST EZ EFI, and we wanted to see if those praises are warranted. According to the guys at FAST, you don't need a laptop or EFI tuning experience; the EZ module handles that for you.

In the past, a carburetor-to-EFI conversion usually resulted in a nightmare of issues like not enough fuel pressure, and tuning. The guys at FAST tell us that with their EZ kit, you simply hook it up, answer the basic setup questions on the included hand-held display, and the system tunes itself while you drive!

The EZ-EFI system provides you with the complete EFI package, including the ECU, wide-band O2 sensor, wiring harness, fuel injectors, an optional fuel pump kit, and other assorted components, including the 4150-base throttle body. If you're running a, shall we say, more aggressive application, the EZ-EFI dual quad upgrade kit is capable of supporting 1,000-plus horsepower engines with dual quad carburetor-type manifolds.

Regardless of your engine size, this single throttle body system bolts-on to any carbureted engine with up to 600 hp-no manifold replacement is needed. The EZ-EFI system also works with your original carburetor style throttle linkage and is ready to accept all OEM sensors. The guys at FAST tell us that the EZ-EFI can be installed easily in an afternoon.

But we weren't going to make our testing of it easy. We didn't want to bolt this thing onto some restored, stock-engined muscle car and go. Nope, that would be too easy. So, we found a car that would definitely test the limits of the EZ kit. Darren Tedder of Prism Racing in Conyers, Georgia, had a customer with a big-inch Hemi that pushed the limits of what some would call a street car, and we felt this would be the perfect test vehicle. The car is a '71 Road Runner with a big Hemi under the hood. We say big because this Hemi displaces 557 inches via a 4.500-inch crankshaft, 7.100-inch connecting rods, and a 4.440-inch bore. Despite its large displacement, the Hemi features a final compression ratio 10.0:1. The camshaft is a flat-tappet design with 248 degrees of duration at .050 inch, and a 110 LSA. The induction is a Ray Barton intake with an 1150 Dominator. When spinning the lie detector, the carbureted engine made 810 hp. Subtle it ain't.